Design of a PMSM Field-Oriented Control Algorithm with Flux-Weakening for Battery Electric Vehicles

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POLITECNICO DI TORINO

DEPARTMENT OF CONTROL AND COMPUTER ENGINEERING Master’s Degree in Mechatronic Engineering

MASTER’S DEGREE THESIS

Design of a PMSM Field-Oriented Control Algorithm with Flux-Weakening for Battery Electric Vehicles

Supervisor:

Prof. Massimo Violante

Candidate:

Riccardo Rossi

Academic Year 2018-2019

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A Carlotta e alla mia Famiglia

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Index

Introduction ... I List of abbreviations ... III

CHAPTER I

Electric and hybrid-electric vehicles

1.1 Automotive applications of electric motors ... 1

1.1.1 History of electric and hybrid-electric vehicles ... 4

1.1.2 Advantages and disadvantages of electric powertrains ... 6

1.2 Electric vehicles ... 8

1.2.1 Energy sources ... 9

1.2.1.1 Batteries ... 10

1.2.1.2 Fuel-cells ... 13

1.2.1.3 Ultracapacitors... 15

1.2.1.4 Ultra-high-speed flywheels ... 16

1.2.1.5 Fundamentals of regenerative braking ... 17

1.2.2 Battery electric vehicles ... 18

1.2.3 Fuel-cell electric vehicles ... 22

1.3 Hybrid-electric vehicles ... 24

1.3.1 Classification of the powertrains ... 26

1.3.1.1 Series hybrid ... 26

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1.3.1.2 Parallel hybrid ... 28

1.3.1.3 Series-parallel hybrid ... 29

1.3.2 Levels of hybridization ... 31

1.3.2.1 Micro hybrid ... 31

1.3.2.2 Mild hybrid ... 31

1.3.2.3 Full hybrid ... 32

1.4 Plug-in hybrid-electric vehicles ... 33

1.5 Controller area network (CAN) protocol ... 34

CHAPTER II Electric AC machines

2.1 Introduction to electric machines ... 41

2.2 Rotating magnetic field ... 49

2.3 Asynchronous motor ... 52

2.4 Synchronous motor ... 55

2.4.1 Permanent magnet synchronous motor (PMSM) ... 63

2.4.1.1 Surface-mounted magnets and interior magnets ... 64

2.4.1.2 Dynamic equations of the PMSM ... 66

CHAPTER III Control applications for PMSMs

3.1 Inverter ... 75

3.1.1 Voltage source and current source inverters ... 78

3.1.2 Modulation strategies for three-phase inverters ... 79

3.1.2.1 Six-step modulation ... 80

3.1.2.2 Pulse-Width Modulation (PWM) ... 82

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3.1.2.3 Space Vector Pulse-Width Modulation (SVPWM)... 87

3.2 Scalar control (V/f) ... 90

3.3 Vector control (Field-Oriented Control) ... 92

CHAPTER IV Development environment

4.1 LabVIEW ... 97

4.1.1 LabVIEW FPGA ... 100

4.1.2 LabVIEW Real-Time ... 104

4.1.3 Example: blinking red LED ... 105

4.2 Hardware platform: SPARK control prototyping unit ... 107

4.2.1 Field programmable gate array (FPGA) ... 110

CHAPTER V Project of the FOC algorithm for a PMSM

5.1 Presentation of the system ... 113

5.2 FOC strategy for an IPM synchronous motor ... 116

5.2.1 Clarke-Park transformations ... 118

5.2.2 Plant implementation ... 123

5.2.3 PI regulators, saturation blocks and anti-windup ... 126

5.2.4 Torque control loop and MTPA region ... 130

5.2.5 Speed control loop ... 135

5.2.6 Flux-weakening for an IPMSM ... 137

5.2.7 Battery pack direct current regulation ... 143

5.2.8 Real-time interface and results ... 147

5.2.8.1 First case: torque loop ... 150

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5.2.8.2 Second case: speed loop and regenerative braking ... 152

5.2.8.3 Third case: battery DC limitation in torque loop ... 154

5.3 FOC strategy for a SPM synchronous motor ... 155

5.3.1 Differences with respect to IPM control algorithm ... 159

5.3.1.1 Decoupling ... 159

5.3.1.2 MTPA region ... 161

5.3.1.3 Flux-weakening for a SPMSM ... 162

5.3.2 Real-time interface and results ... 163

5.4 BMS and dashboard simulation using CAN protocol ... 164

Conclusion ...171

Bibliography ...174

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I

Introduction

The thesis is focused on the design of a field-oriented control algorithm with flux- weakening for a permanent magnet synchronous motor (PMSM). The development of the controller is based on LabVIEW FPGA language, using a model-based approach to create the block scheme and to run the autogenerated code on the SPARK engine control unit of Alma Automotive. The realized control strategy takes into account possible automotive applications of this kind of electric motors, discussing and facing in particular the issues related to battery electric vehicles (BEVs). For the project, both interior permanent magnet and surface-mounted permanent magnet synchronous machines have been considered, analysing the behaviour of the first typology with torque loop and speed loop control mechanisms, while for the second one just the torque-based strategy is implemented. The plant and controller models are flashed on the field programmable gate array integrated circuit of the board, while the user interface – with all the commands and the programmable parameters – exploits the hardware real-time module. In the final part of the project, the battery management system and the dashboard are simulated, and the CAN communication protocol is included in order to show the obtained results.

In the first part, electric and hybrid-electric vehicles are analysed, classifying and evaluating them according to their powertrain systems and hybridization levels. An historical description of development and diffusion of these cars is provided, considering also their presence on the worldwide market nowadays. A short presentation of CAN bus is inserted for explaining its features and for describing its use in automotive applications. In the second chapter, the attention is focused on the electric AC machines: the discussion starts from their basic physical principles and their comparison with DC typology. For a complete analysis, asynchronous and synchronous motors are described, studying in particular the dynamic equations of the PMSM. In that section the mathematical models are reported, in order to explain the main properties of FOC algorithm. For what concerns the driving strategy, after an introduction about the inverters and their possible implementation and modulation mechanisms, the scalar regulation and the field-oriented control are presented to show the

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differences among them. In the fourth chapter the practical aspects for the control development are introduced: the LabVIEW FPGA – used for the realization of the models – and LabVIEW Real-Time – for the creation of the user interface – modules are described together with the SPARK control prototyping system, concentrating the attention on the main features that are useful for the design. A short description of field programmable gate array system is included too.

The final chapter regards the implementation of the block diagrams for both controller and plant, reporting the relevant schemes and the final results obtained through the simulation of interior permanent magnet and surface-mounted permanent magnet synchronous motors. Moreover, in this part the practical aspects and the faced problems are considered, focusing the analysis on the adopted solutions and the optimization strategies of the algorithm. For a complete scenario of the possible automotive application, also the CAN communication is implemented in the real-time interface, in order to exchange messages with simulated nodes – in particular with the dashboard control unit and the battery management system, for analysing the use of the designed algorithm in a battery electric vehicle. The project has been developed in collaboration with the Automotive area of Teoresi Group S.p.A. in Turin, from March 2019 to October 2019.

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III

List of abbreviations

ABS – Anti-lock Braking System AC – Alternating Current

ASIC – Application-Specific Integrated Circuit

BC – Black Carbon

BCM – Body Computer Module BEV – Battery Electric Vehicle BJT – Bipolar Junction Transistor BLDC – Brushless DC motor

BMS – Battery Management System CAN – Controller Area Network

CAN FD – Controller Area Network with Flexible Data-rate CHG – Compressed Hydrogen Gas

CO² – Carbon Dioxide

CPU – Central Processing Unit CSI – Current Source Inverter DC – Direct Current

DSP – Digital Signal Processing ECU – Engine Control Unit

EEPROM – Electrically Erasable Programmable Read-Only Memory EMF – ElectroMotive Force

EPROM – Erasable Programmable Read-Only Memory ESP – Electronic Stability Program

EV – Electric vehicle

FCEV – Fuel-Cell Electric Vehicle FIFO – First-In First-Out

FOC – Field-Oriented Control

FPGA – Field Programmable Gate Array

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FW – Flux-Weakening FXP – Fixed-Point

HDL – Hardware Description Language HEV – Hybrid-electric vehicle

HIL – Hardware-In-the-Loop

I/O – Input/Output

ICE – Internal Combustion Engine IG – Interactive Generator

IGBT – Insulated Gate Bipolar Transistor IM – Induction Motor

IPM – Interior Permanent Magnet

IPMSM – Interior Permanent Magnet Synchronous Motor LED – Light Emitting Diode

Li-ion – Lithium-ion battery LUT – Look-Up Table MMF – MagnetoMotive Force

MOSFET – Metal-Oxide-Semiconductor Field-Effect transistor MTPA – Maximum Torque Per Ampere

MTPV – Maximum Torque Per Voltage Ni-MH – Nickel-Metal Hybride battery NOx – Nitrous Oxides

PEMFC – Proton Exchange Membrane Fuel-Cell PHEV – Plug-in Hybrid Electric Vehicle

PI – Proportional Integral controller/regulator PM – Permanent Magnet

PMSM – Permanent Magnet Synchronous Motor PROM – Programmable Read-Only Memory PWM – Pulse-Width Modulation

RAM – Random-Access Memory

RF – Reference Frame

RIO – Reconfigurable Input/Output RMF – Rotating Magnetic Field RPM – Revolutions Per Minute

RT – Real-Time

SM – Synchronous Motor

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V SOC – State Of Charge

SPFC – Solid Polymer Fuel-Cell

SPM – Surface-mounted Permanent Magnet

SPMSM – Surface-mounted Permanent Magnet Synchronous Motor SRAM – Static Random-Access Memory

SRM – Switched Reluctance Motor

SVPWM – Space Vector Pulse-Width Modulation VI – Virtual Instrument

VSI – Voltage Source Inverter ZEV – Zero-Emission Vehicle

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Chapter I:

Electric and hybrid-electric vehicles

SUMMARY: 1.1 Automotive applications of electric motors – 1.1.1 History of electric and hybrid- electric vehicles – 1.1.2 Advantages and disadvantages of electric powertrains – 1.2 Electric vehicles – 1.2.1 Energy sources – 1.2.1.1 Batteries – 1.2.1.2 Fuel-cells – 1.2.1.3 Ultracapacitors – 1.2.1.4 Ultra-high-speed flywheels – 1.2.1.5 Fundamentals of regenerative braking – 1.2.2 Battery electric vehicles – 1.2.3 Fuel-Cell electric vehicles – 1.3 Hybrid-electric vehicles – 1.3.1 Classification of the powertrains – 1.3.1.1 Series hybrid – 1.3.1.2 Parallel hybrid – 1.3.1.3 Series-parallel hybrid – 1.3.2 Levels of hybridization – 1.3.2.1 Micro hybrid – 1.3.2.2 Mild hybrid – 1.3.2.3 Full hybrid – 1.4 Plug-in hybrid-electric vehicles – 1.5 Controller area network (CAN) protocol.

1.1 Automotive applications of electric motors

In recent years, electric and hybrid-electric vehicles development and control techniques are advancing, with important changes and innovations for what concerns the powertrain, the driver assistance systems of modern cars and their automation. The automotive industry is experiencing a global renovation, especially for the reduction of fuel consumption and pollution: in fact, many companies are actually decreasing the production of internal combustion engine cars to concentrate their efforts on the research of modern technologies to improve the environmental sustainability. Consequently, the industry is expending considerable efforts and resources to address the challenges associated with developing alternative solutions for transportation, reducing the emission of dangerous substances – that influences the air quality – and the dependence on petroleum products and non-renewable resources.1

1 Khajepour, A., Fallah, S., & Goodarzi, A. (2014), Electric and Hybrid vehicles. Technologies, Modeling and Control: A Mechatronic Approach, Chichester, West Sussex, United Kingdom: Wiley, p.1.

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«Electric vehicles (EVs) and hybrid-electric vehicles (HEVs) present many and complex design problems that are absent in the well-established conventional automotive technologies and transportation systems».² In order to counterbalance their drawbacks, the design of efficient and long-range cars is one of the major issues that automotive engineers are facing nowadays. For what concerns the electric powertrain system, the development of new powerful motors is fundamental for the increase of vehicles production, considering also the modern control algorithms that assure more reliability and efficiency for their use.

Electric, hybrid-electric and fuel-cell-powered drivetrain technologies are the most promising vehicle solutions to replace conventional vehicles in near future.3

During past decades – when vehicles became accessible for all the citizens – the internal combustion engine has dominated the transportation industry and today it can be considered an essential part of human life. In fact, the ICE powertrain is actually perfectly known and optimized, while efficient simulation tools are used to describe its behaviour.⁴ The combination of this powertrain with the transmission system – manual, automatic, automated or continuous – allows adapting the engine speed to the vehicle velocity: for example, in the manual one a set of gears can be shifted directly by the driver through a clutch pedal and a shift knob. This system operates safely with a high power to weight ratio.

Moreover, an ICE vehicle can be also recharged in a very fast way; in fact, it’s difficult to overcome the forms of energy that have the highest density and are easily accessible, like gasoline and diesel. The same cannot be said for battery-electric components, because an efficient infrastructure to support the demand of power stations doesn’t already exist.

Despite the evolution of modern ICEs, some disadvantages are inevitably related to these systems: for example, the impossibility of generating torque from rest, the losses due to heat, the incomplete combustion and the friction between moving parts of the system.

Practically, the real efficiency of this engine is around 35% for modern vehicle, because the remaining quantity is dissipated through the exhaust and cooling systems. A transmission mechanism is needed to connect the ICE to the driving wheels, increasing in this way the number of moving elements and consequently the friction losses and the overall inertia; also noises and vibrations are negative aspects that affect internal combustion engines and not the electric motors.

2 Khajepour, A., Fallah, S., & Goodarzi, A. (2014), op. cit., p. 47.

3 Ehsani, M., Gao, Y., & Emadi, A. (2010), Modern Electric, Hybrid Electric, and Fuel Cell Vehicles.

Fundamentals, Theory, and Design (Second edition), Boca Raton, Florida, United States: CRC Press, p. 1.

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Figure 1.1: Engine torque - vehicle velocity characteristics of an ICE car.⁵

Obviously the most evident drawback is the emission of exhaust gas from the vehicles and the use of petroleum products as fuel; «conventional cars with internal combustion engines (ICE) are still a major source of air pollutants such as carbon dioxide (CO²), nitrogen oxides (NOx), black carbon (BC) and fine particulate matter».⁶ Moreover, the diesel emission scandal – that started in 2014, involving all the main car manufacturers for the exceeding of the legal European emission limits for nitrogen oxide by more than ten times – has given a further incentive to the development of electric vehicles. For all these reasons, the electric powertrains – applied in the automotive sector – are actually experiencing a substantial growth in terms of utilization, research and development.⁷ However, the adoption of this kind of engine has only been rediscovered and improved, because the concept of electric vehicles preceded the diffusion of ICEs. The advantages of electricity for mobility applications have been studied since their introduction in the 19^th century and their improvement at the beginning of the 20^th, during the so-called Golden Age.

5 Velardocchia, M. (2019), Fundamentals of Longitudinal Dynamics (Technologies for Autonomous Vehicles), Turin, Italy: Politecnico di Torino, p. 18.

6 Buekers, J., Van Holderbeke, M., Bierkens, J., & Int Panis, L. (2014, December), Health and environmental benefits related to electric vehicle introduction in EU countries, In Transportation Research Part D: Transport and Environment, 33, p. 26.

7 Bayindir, K., Gözüküçük, M., & Teke, A. (2011, February), A comprehensive overview of hybrid electric vehicle: Powertrain configurations, powertrain control techniques and electronic control units, In Energy Conversion and Management, 52(2), p. 1311.

Longitudinal dynamics - driving

Output converter:

• speed converter: mechanical or hydrodynamic clutch

• speed-torque converter: geared transmission or continuously

variable transmission

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4

1.1.1 History of electric and hybrid-electric vehicles

The electric vehicle has been one of the first cars to be invented around 1835 in the Netherlands by Sibrandus Stratingh and Christopher Becker, who created a small-scale electric car, powered by non-rechargeable primary cells. The evolution of batteries to store energy gave a significant improvement to the manufacturing of electric-based solutions for the mobility, in terms of vehicles and locomotives. English inventor Thomas Parker, responsible also for the electrification of London Underground, built the first production electric vehicle in 1884 in London, using rechargeable batteries; he was interested in fuel- efficient vehicles to reduce the malign effects of smoke and pollution in the city. Moreover, France, United Kingdom, Germany and the United States supported the widespread development of this electric cars and trains. The most significant technical advance of that era was the invention of the regenerative braking by M. A. Darraq in 1897: this method enhances the driving range, recuperating the vehicle’s kinetic energy while braking and recharging the batteries. This can be defined as one of the major contributions to electric and hybrid-electric vehicles’ efficiency.⁸

At the beginning of 20^th century – during the Golden Age – electric vehicles obtained many speed and distance records: for example, in 1899 “La Jamais Contente”, built by Frenchman Camille Jenatzy, was the first ever vehicle to reach 100 km/h.⁹ EVs had many advantages with respect to first gasoline-based competitors: less vibration, smell and noise and obviously no gear shifting, giving the perception of very simple vehicles. These cars were preferred because they not required a manual effort to start, differently from gasoline cars which needed a hand crank to start the engine; moreover, many homes were wired for electricity in the early 1910s, increasing the popularity of electric solutions, that reach the peak production in 1912. In the USA in that period the 38% of registered vehicles were powered by electricity.

After the success of the “Golden Age” the decline of electric cars began in the 1920s due to the improvement of road infrastructure: the limitation of the autonomy was the biggest problem, because at the same time gasoline cars became more powerful, more flexible and they guaranteed the possibility of travelling faster and further.¹⁰ The beginning of mass production brought the price of gas-powered vehicles down, while the price of electric cars

8 Ehsani, M., Gao, Y., & Emadi, A. (2010), op. cit., p. 13.

9 Ibidem.

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continued to rise. The union of all these aspects caused the disappearance of electric automobile industry from the 1920s – except for some restricted applications (as golf carts and delivery vehicles) – and the internal combustion engine started to dominate the market, as well as nowadays. Moving forward in time, scientists began to worry about the environmental effects of ICE exhaust emissions. For this reason – combined with the fluctuations of hydrocarbon energy market – from the 1970s governments started to take several legislative and regulatory actions to moderate oil production dependency. In this way the conditions for the development of alternatively-powered vehicles were created, so many manufacturers renewed their attempts to produce electric cars. In 1974, The American company Sebring-Vangaurd designed the CitiCar, the first mass-produced electric vehicle.

In that period, General Motors Company, Renault, Peugeot and Audi spent a massive effort for electric powertrain development and research.11

However, the electric vehicles market failed in the 1990s due to the limited autonomy and performance of these cars with respect to gasoline-powered ones. Moreover, the EVs’

high initial and maintenance costs contributed to their commercial failure. Finally, from the 2000s many highway-capable electric vehicles arrived on the market: for example, in 2004 Tesla Motors began to develop the Tesla Roadster that was able to travel more than 320 km per charge, using lithium-ion battery cells. The demand of sustainable solutions to reduce the emissions of ICE vehicles – in particular the diesel ones – led to the diffusion of today electric cars: General Motors, Nissan, Tesla and Toyota, and all the main automakers from the 2010s began to produce zero-emission vehicles, using new technologies to increase their autonomy or to regenerate energy.

Another fundamental aspect was the appearance of hybrid cars on the market in the late 1990s, combining internal combustion engine with an electric motor. Toyota launched successfully the Prius in Japan in 1997, while the Honda Insight arrived on the market in 1999; in 2004 Ford produced the first hybrid SUV. These vehicles were the starters for HEVs’ diffusion:¹² modern hybrid vehicles guarantee low emissions, high fuel economy and obviously performance comparable to ICEVs. They represent the perfect solution towards the full electrification of the mobility. HEVs reached success especially in Japan, which surpassed the United States as world’s largest hybrid market in 2014. Nevertheless, the diffusion of EVs and HEVs is actually quite small if compared with ICE cars: every aspect

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6 of human life and energy distribution is still based on petroleum products and the infrastructure are not enough adequate to support the demand of recharging stations.

1.1.2 Advantages and disadvantages of electric powertrains

From the previous historical analysis, the main advantages of the electric powertrain can be defined. First of all, the efficiency is huge with respect to ICE and it can reach peaks over 85%. The gearbox is no more needed because electric engines can operate at significantly high angular speed: consequently, an electric car has only one gear to connect the engine to the shafts. This typology of motor operates safely over a wide range of vehicle speed, guaranteeing always a high traction torque. Obviously, a fundamental advantage is the absence of emissions and the possibility to reduce air pollution. As described before, electric powertrain has the capability of generating instant torque from rest without using any starters, and it’s quite lighter and smaller if compared to internal combustion engine;

moreover, this motor can reach the idle state and maintain the zero-speed condition, so it doesn’t rotate if it is not necessary – for example when the vehicle is stopped.

An electric motor is able to rotate in both directions – clockwise and anticlockwise – so the vehicle can go back and forth, without the need of a reverse gear system. The transmission can be further simplified, reducing the weight and the heat losses due to friction. The electric machine can also operate as generator, when torque and speed have opposite signs. This benefit is exploited for recovering the kinetic energy with the regenerative braking and the potential energy during downhill driving, in order to recharge the energy source and increase the autonomy range. The following figure 1.2 shows the four operational modes – the four quadrants – of this kind of machines. The use of external electric grid for recharging these vehicles is generally less costly than other fuels; moreover, maintenance and repair costs are estimated to be lower with respect to conventional gasoline vehicles. Obviously, the remarkable initial cost is seen as an obstacle for electric-based cars’

diffusion.

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Figure 1.2: Four-quadrant operation modes of the electric machine (motor torque - mechanical speed characteristics).13

Regarding the application of the electric powertrains and the diffusion of HEVs and EVs, some interesting questions arise analysing their history, their success and their failure during the last century. In particular, the main problem is the origin of the electricity:

actually, its production is still a relevant cause of pollution. Consequently, with EVs and HEVs the damages to the environment would be only partially reduced. The development of power stations based on renewable resources (solar energy, wind, water, geothermal energy) will consistently improve the air quality. The diffusion of this kind of vehicles cannot be split from the research of new solutions for the production of green energy.¹⁴

Secondly, one of the adopted solutions for energy storage is the use of batteries, in particular the lithium-ions cells – energy sources management will be analysed in next paragraphs. These elements are considered hazardous and they must operate in a specific condition of equilibrium, from electric and temperature points of view. These batteries are obviously subject to degradation and they must be replaced after a certain amount of time, because their capability is reduced. Consequently, the disposal is a critical aspect that is actually the argument of many studies:¹⁵ the continuous development of new technologies

13 Wei, L. (2017), Hybrid electric vehicle system modeling and control (Second Edition), Chichester, West Sussex, United Kingdom: Wiley, p. 148.

14 Buekers, J., Van Holderbeke, M., Bierkens, J., & Int Panis, L. (2014, December), op. cit., p. 35.

15 Gaines, L. (2014, December), The future of automotive lithium-ion battery recycling: Charting a sustainable course, In Sustainable Materials and Technologies, 1-2, p 2.

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8 implies that the batteries are difficult to be reused and recharged; moreover, the extraction of lithium from old ones is five times more expensive than mined lithium.

Finally – as considered before – the autonomy of this kind of vehicles is an essential problem that has always influenced their history and their diffusion: nowadays the electric vehicles cannot compete with the ICE ones or the hybrid cars, because the mileage is sufficient only for city driving. The mechanism of idle-off and the regenerative braking system are fundamental aspects to increase the autonomy.16 The diffusion of charging stations will also greatly improve the use of electric vehicles, but the time needed for a complete recharge is obviously higher with respect to gasoline-based vehicles. An additional issue is the absence of noise that can be seen as an advantage from the driver and passengers’

point of view, but as a danger for the pedestrians and other drivers; in many cases, sensors are mounted on these vehicles to advice surrounding people and cars.

In the following paragraphs, the main characteristics and the classification criteria of both electric and hybrid-electric vehicles will be analysed.

1.2 Electric vehicles

An electric vehicle – referred to as an electric drive vehicle – uses one or more electric motors for propulsion. An electric vehicle can be self-contained with battery or it can be powered by off-vehicle sources, depending on the application. EVs include road and rail vehicles, surface and underwater vessels and electric aircraft and spacecraft; despite the domination of ICE for almost 100 years, electric power remained fundamental for other mobility solutions, like trains and smaller vehicles. Recently, electric cars saw a fast and crucial development, thanks to the augmented focus on renewable energy and limitation of air pollution. From 2018 on the market there are almost 180 models of highway-capable all- electric passenger cars: Nissan Leaf is the world’s top selling rechargeable electric car, followed by Tesla Model S and Model 3. To enlarge the diffusion of electric vehicles, several countries have recently established grants and subsidies for the purchase of the vehicle and for the advancement of the studies on cars and batteries. Electric vehicles are expected to increase from 2% of global share in 2016 to 22% in 2030, together with the development of new and more adequate infrastructure for energy distribution and more efficient mechanism for energy storage. «The price of the EVs is quite high compared to their ICE counterparts.

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This is because of the high cost of batteries and fuel-cells. To make people overlook this factor, governments in different countries including the UK and Germany, have provided incentives and tax breaks which provide the buyers of EVs with subsidies. Mass production and technological advancements will lead to a decrease in the prices of batteries as well as fuel-cells».17

1.2.1 Energy sources

In contrast to vehicles powered by conventional ICE, energy storage systems are of crucial importance for electric vehicles.¹⁸ Different technologies have been proposed and studied in recent years to guarantee the needed supply for the electric powertrain: batteries, fuel-cells, ultracapacitors and flywheels are the main adopted solutions. Batteries, capacitors and flywheels are systems where the electrical energy is stored during charging, while fuel- cells are energy generation systems based on chemical reaction. Actually, batteries are the fundamental elements of modern EVs because of their reasonable cost and their technological evolution.¹⁹ Differently from ICE’s high performance, these energy sources cannot provide high specific energy and high specific power simultaneously; the specific energy is the energy capacity per unit source weight, while the specific power is defined as the maximum power per unit energy source weight.²⁰ For this reason, hybridization techniques are actually being studied for combining two or more storages together so that the disadvantages of each can be compensated by others and the positive aspects can be exploited.²¹ Basically, one energy storage provides high specific energy and the other high specific power. For high power demand operations, like acceleration or hill climbing, both basic energy sources deliver their power. In low power demand situations, like constant

17 Un-Noor, F., et al. (2017, August), A Comprehensive Study of Key Electric Vehicle (EV) Components, Technologies, Challenges, Impacts, and Future Direction of Development, In Energies, 10(8), p. 58.

18 Eberle, U., & von Helmolt, R. (2010), Fuel Cell Electric Vehicles, Battery Electric Vehicles, and their Impact on Energy Storage Technologies: An Overview, In G. Pistoia, Electric and Hybrid Vehicles. Power Sources, Models, Sustainability, Infrastructure and the Market (pp. 227-245), Amsterdam, The Netherlands: Elseiver, p. 227.

19 Chau, K., Wong, Y., & Chan, C. (1999, July), An overview of energy sources for electric vehicle, In Energy Conversion & Management, 40(10), p. 1023.

20 Ehsani, M., Gao, Y., & Emadi, A. (2010), op. cit., pp. 380-384.

21 Chau, K., Wong, Y., & Chan, C. (1999, July), op. cit., p. 1035.

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10 speed operations, the high specific energy storage delivers its power to the load and charge also the high specific power storage. In case of negative power – regenerative braking situation – the peak power is absorbed by the high specific power element and only partially by the other.22

Figure 1.3: Hybridization concept in different operating conditions (a) hybrid powering, (b) power split, (c) hybrid recharging.23

1.2.1.1 Batteries

Battery systems are expected to be a safe and reliable source of energy that delivers the high performance that modern battery cells and chemistries have to offer.

Electrochemical batteries are electrochemical device that convert electrical energy into potential chemical energy during charging and convert the stored chemical energy into

23 Ivi, p. 405.

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electric energy during discharging. Batteries are composed by several cells stacked together:

each cell is an independent unit composed by two electrodes (positive and negative) immersed into electrolyte.²⁴

Figure 1.4: Electrochemical battery cell.25

One of the main parameters that are specified by manufacturers is the coulometric capacity (ampere-hours) that defines the number of ampere-hours gained when discharging the battery from a fully charged state until the terminal voltage drops reaches its cut-off voltage. Usually the discharging current rate influences the capacity – for example a large discharge rate causes a smaller capacity – so the manufacturers specify the discharge rate for a certain capacity. Another important value is the state of charge (SOC) of a battery, defined as the ratio of remaining capacity to fully charged capacity. The thermodynamic voltage of a battery cell is closely associated with the energy released and the number of electrons transferred in the chemical reaction; on the other hand, the efficiency expresses the cell operating voltage to the thermodynamic voltage, for evaluating the energy or power losses in terms of voltage losses. Finally, specific energy and specific power are two essential parameters for evaluating the performance of a battery.26

25 Ivi, p. 376.

26 Ibidem.

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12 Different technologies have been used for developing adequate batteries for electric and hybrid-electric vehicles; the principal three solutions are: lead-acid batteries, nickel- based and nickel metal hybride batteries, lithium-ion batteries.²⁷ Lead-acid battery is the oldest rechargeable one and it has represented a successful commercial product for over a century; it’s still widely used as electrical energy storage for many automotive applications, for example for starting the engine in conventional vehicles.²⁸ This technology requires lower cost compared with other solutions, high cell open-circuit voltage, easy recycling and accurate SOC indication. The principal drawbacks are related to the low energy density and short life cycle, high self-discharge rate and low charge/discharge efficiency.29 Thanks to their high power, these energy storage systems are attractive for HEV applications; new lead-acid are actually being developed for improving the performance of electric and hybrid- electric cars: recently, the main disadvantages – as low specific energy, reduced lifetime and safety issue – have been overcome.

Nickel is a lighter metal than lead and guarantees good electrochemical properties for battery applications. Nickel/iron was the first nickel-based solution, but it suffered from gassing, corrosion and self-discharge problems, with a non-negligible initial cost; on the other hand, the higher power density with respect to lead-acid batteries was a huge benefit.

Then nickel/cadmium batteries have been introduced, guaranteeing an enormous technical improvement for their high specific power, long life cycle, rapid charge capability, negligible corrosion and low self-discharge rate. The initial cost is still a problem, together with the environmental hazard due to cadmium: despite its recyclability, it can cause pollution if not properly disposed of.30 A nickel-metal hybride (Ni-MH) battery uses hydrogen absorbed in a metal alloy for the active negative material; since a metal hybride electrode has a higher energy density than a cadmium electrode, this battery has higher capacity and longer life with respect to previous nickel-based technologies.31 Ni-MH battery is free from toxicity or carcinogenicity – differently from cadmium – so it is actually a suitable choice for EV and HEV applications³²; in fact, Ni-MH batteries are used for most

29 Wei, L. (2017), op. cit., p. 25.

31 Wei, L. (2017), op. cit., p. 26.

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HEVs currently sold in the United States.33 As drawbacks, limitations in energy and power density are not negligible, while the initial cost is an obstacle for their diffusion.

The lithium-ion (Li-Ion) battery is actually viewed as the battery that guarantees the long-term goals for EVs. It uses lithiated carbon intercalation material for the negative electrode, lithiated transition metal oxide for the positive electrode and liquid organic solution for the electrolyte: lithium ions are extracted from the negative electrode and inserted to the positive one on discharge and vice versa on charge operation. 34 This kind of battery exhibits high specific energy and specific power together with a long cycle life; the main drawback is related to the huge initial cost.35

New battery technologies will also create a need for new management capabilities;

moreover, new active materials are continuously developed for improving energy capacity, cycle life and autonomy range.36 Lithium-ion batteries are incorporated into advanced devices, from automotive to aviation, aerospace and defence, so a higher level of reliability and safety must be assured, reducing the risk of errors concerning the production and the operational mode of these batteries. «The field of Li-Ion solutions for energy storage is a fertile area for many years to come, leading to new advances in the fields of transportation and energy storage»37: management systems will need to cope with significant improvement in battery behaviour, maintaining always an accurate prediction of the battery condition and assuring the high reliability that all electronic control systems must guarantee.

1.2.1.2 Fuel-cells

Fuel-cells – and also ultracapacitors – have received attention recently and they have showed optimal performance in the mid-term.³⁸ Fuel-cells energy storage system is based on the conversion of the hydrogen and oxygen chemical energy directly into electrical

33 Axsen, J., Burke, A. F., & Kurani, K. S. (2010), Batteries for PHEVs: Comparing Goals and the State of Technology, In G. Pistoia, Electric and Hybrid Vehicles. Power Sources, Models, Sustainability, Infrastructure and the Market (pp. 405-427), Amsterdam, The Netherlands: Elseiver, p. 418.

35 Weicker, P. (2014), A systems approach to Lithium-Ion Battery Management, Norwood, Massachusetts, United States: Artech House, p. 25.

36 Axsen, J., Burke, A. F., & Kurani, K. S. (2010), op. cit., p. 420.

37 Weicker, P. (2014), op. cit., p. 23.

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14 energy. In this case, the electricity-producing reactants are continually supplied from external sources to the fuel-cell itself: oxygen from the air and hydrogen from a separate onboard tank. Differently from batteries, the reactants are not regenerated by the recharging process.39

Solid polymer fuel-cells (SPFCs) – called also proton exchange membrane fuel-cells (PEMFCs) – are actually the most studied and developed technology for EVs. SPFC uses a solid polymer membrane as the electrolyte and has the advantage of the highest power density;40 however, the cost of the catalyst is significant compared to ICE, because noble metals are needed due to low operating temperature of the fuel-cell and the acid nature of the electrolyte. Other critical issues are poisoning problem and water management because polymer membrane needs to be kept humid, otherwise there will not be enough acid ions to carry the protons; on the other hand, a too much wet membrane blocks reactant gases.41 This technology guarantees advantages that motivate their future diffusion, such as low- temperature operation, fast start-up, high power density and reduced dimension.

Alternative fuel-cells are the alkaline ones, based on an aqueous solution of potassium hydroxide as electrolyte to conduct ions between electrodes, that are cheaper and guarantee high efficiency, but with a limited durability due to corrosive electrolyte.42

Phosphoric acid fuel-cells are similar to PEMFCs, because they use an acid electrolyte to conduct hydrogen ions; they work at low temperature and guarantee fast start-up, but they need expensive catalyst and are subject to corrosion and carbon-monoxide poisoning.⁴³ Finally, molten carbonate fuel-cells work at high temperature and use a molten carbonate salt to conduct ions: differently from above technologies, these cells are supplied directly by hydrocarbons because the high temperature allows their decomposition to hydrogen on the electrodes. These fuel-cells require also low-cost catalyst and have improved efficiency, but they are slower and need only a particular set of materials due to high temperature.44

39 Delucchi, M., & Lipman, T. (2010), Lifetime Cost of Battery, Fuel-Cell, and Plug-in Hybrid Electric Vehicles, In G. Pistoia, Electric and Hybrid Vehicles. Power Sources, Models, Sustainability, Infrastructure and the Market (pp. 19-60), Amsterdam, The Netherlands: Elseiver, p. 45.

43 Ivi, p. 447.

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Figure 1.5: Schematic of a SPFC (or PEMFC).45

For hydrogen storage, the main adopted methods are three: storing it as a compressed gas (compressed hydrogen gas, CHG), so pure hydrogen is stored onboard the vehicle under pressure in a tank; in form of liquid hydrogen in cryogenic containers, a very hazardous application; using the metal hybrid technology, so hydrogen reacts with some metals like magnesium and vanadium. CHG solution and metal hybrids are still huge challenges for vehicle application, but they can guarantee optimal performances.⁴⁶

1.2.1.3 Ultracapacitors

The difficulty in simultaneously obtaining high values of specific energy, specific power and cycle life has led to hybridization techniques. Fuel-cells and batteries have high specific energy, so a power source with high specific power is needed, and ultracapacitors received huge attention for this purpose.47 These elements – based mainly on double-layer capacitor technology – are characterized by a specific power that can reach up to 3 kW/kg,

46 Ibidem.

47 Ivi, p. 1032.

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16 much higher than any battery – so they can provide high output power within a short period of time, in case of huge power demand conditions.48

Moreover, the life cycle of batteries is affected by frequent start/stop operations of electric powertrain and by high peak power demands, jeopardizing the efficiency of regeneration system: ultracapacitors are able to minimize the huge discharging current and also the high charging current which arrives to the battery through the regenerative braking system, so the available energy, endurance and life of the battery can be greatly increased.49

Therefore, the hybridization technique allows providing optimal energy recovery. On the other hand, ultracapacitors’ implementation requires additional power electronics, increasing the initial costs of the vehicle. Actually, this mechanism is not enough powerful to support the power demand of an EV, but its evolution will significantly improve vehicles’

performance. As additional issue, for an efficient hybridization an accurate control of the power flow between ultracapacitors, batteries, motors and power electronics is needed.⁵⁰

1.2.1.4 Ultra-high-speed flywheels

Flywheels are not a new concept: they were introduced around 25 years ago for powering passenger buses, but they were heavy, and their rotor wasn’t fast. Modern flywheels consist of a lightweight composite rotor, able to rotate at the order of ten thousand of revolutions per minute (RPM).51 This energy storage system can achieve the requirements for EV applications, guaranteeing high specific power, high specific energy, high efficiency, quick recharging, reduced costs and almost unlimited life cycle. In case of hybridization, flywheels are used as an auxiliary energy source to store energy in mechanical form during period of cruise speed or regenerative braking. «To charge and discharge the ultra-high- speed flywheel, which is directly coupled to the machine rotor, the permanent magnet (PM) brushless machine has been accepted to be the most appropriate type».⁵² Consequently, many benefits are associated to the use of this storage system, like also rapid short-term recharges and extension of vehicle range.

50 Ibidem.

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Despite the previous description, some problems affect their diffusion: first, the vehicle manoeuvrability could be reduced due to possible gyroscopic phenomena when the vehicle departs from its straight-line trajectory; secondly, in case of malfunctions the stored mechanical energy will be released very rapidly, causing severe damages to the other parts of the system. To overcome this inconvenient, the use of multiple flywheels will reduce the gyroscopic effects, while new safer failure containments are being developed. Ultra-High- speed flywheels are analysed as the main solution for the progress of long-term EVs.53

1.2.1.5 Fundamentals of regenerative braking

Regenerative braking is an energy recovery system that converts the vehicle kinetic energy into a form that can be stored, when the vehicle is braking. In conventional ICEVs, the braking system helps to decelerate or stop a moving vehicle by supplying a sufficient braking torque on all wheels and creating friction between the brake pads and the wheels;

consequently, a substantial amount of energy is dissipated as excessive heat energy.⁵⁴ The electric motors of EVs and HEVs can be controlled to operate as generators to convert the kinetic energy into electric energy that can be stored and reused. This aspect is a unique feature of these vehicles that distinguishes them from gasoline-based cars. In fact, electric machines are able to work as motors or as generators – converting mechanical energy into electric energy55 – depending on the operating mode. The main advantages due to regenerative braking are the improved efficiency and fuel economy, the reduction of brake and engine wear with respect to ICE, the attenuation of emissions thanks to the mitigation of dissipated energy and the increase of the operating range.56

Modern electric motors cannot provide the necessary braking torque in case of heavy braking. For this reason, hybrid braking system are implemented to guarantee optimal performance and to recover as much energy as possible.⁵⁷ When the regenerative braking is at maximum, the additional hydraulic system provides the required torque. In fact,

54 Ivi, p. 411.

55 Bhandari, P., Dubey, S., Kandu, S., & Deshbhratar, R. (2017, February), Regerative Braking Systems (RBS), In International Journal of Scientific & Engineering Research, 8(2), p. 72.

56 Bhandari, P., Dubey, S., Kandu, S., & Deshbhratar, R. (2017, February), op. cit., p. 73.

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18 regenerative braking is necessarily limited when the energy sources are fully recharged. For high-speed applications, regenerative braking doesn’t work efficiently because electric machine – acting as generator – is in constant power region (also called flux-weakening region):58 consequently, the torque capability is lower than the nominal one.

Figure 1.6: Hybridization of regenerative and hydraulic braking system battery cell.⁵⁹

1.2.2 Battery electric vehicles

A battery electric vehicle runs entirely on a battery and electric drivetrain, without a conventional internal combustion engine; BEVs are the simplest type of electric vehicle from a conceptual perspective, because the electrical power of the electrochemical battery is exploited to power one or more electric machine. The typical configuration requires a single motor connected to the front axle through a simple gearbox; another significant variation is to use four hub motors attached to each wheel.⁶⁰ These vehicles must be plugged into an external source of electricity to recharge their batteries; they can be recharged from grid electricity at recharging stations or houses, non-grid sources as solar panels or using recuperative energy systems – as previously mentioned: potentially a BEV can emit zero

59 Ivi, p. 64.

60 Delucchi, M., & Lipman, T. (2010), op. cit., p. 23.

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greenhouse gases and air pollutants. In addition to environmental advantages, electric vehicles guarantee a higher efficiency with respect to internal combustion engine cars;

moreover, the required maintenance is expected to be lower due to the reduction of mechanical and emission control components.61 For example, muffler, tailpipe and gas tank are totally absent from electric vehicles.

Despite the benefits, some disadvantages are still related to BEV. First, the low energy and power density of batteries with respect to liquid fuels, i.e. gasoline or diesel.

Second, once depleted, charging the battery pack takes quite a lot of time compared to refuelling an ICE vehicle – there are less time-consuming solutions, but none is comparable to the little time required to refill a fuel tank. Third, they can cover only 100-250 km on one charge, whereas the top-tier models can reach 500 km; these ranges depend on driving condition and style, vehicle configurations, road conditions, climate, battery type and age.62

Modern BEVs have various configurations: some elements can be present in a vehicle and not in other one, depending on the design and providing consequently different performance in terms of autonomy, traction torque, recharging time, delivered power, weight and dimensions. Typically, the key components of this kind of cars are:

▪ one or more electric motors to convert the electric energy from energy sources into mechanical energy to provide the required traction force for the vehicle motion;63

▪ traction battery pack – regulated by the BMS – to store electricity for the electric motor;

▪ an auxiliary battery for providing the required electricity to power the accessories in the electric drive vehicle;

▪ battery management system (BMS) that is the control unit for monitoring the traction battery pack;

▪ charge port to connect the vehicle to an external supply;

▪ DC/DC converter to convert the higher-voltage DC power from the traction battery to the lower-voltage DC power needed to run vehicle accessories and recharge the battery;

▪ onboard charger that takes the incoming AC electricity via the charge port and converts it to DC power for charging the battery. Moreover, it monitors the battery status and operating condition for guaranteeing the correct current;

62 Un-Noor, F., et al. (2017, August), op. cit., p. 3.

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20

▪ thermal system to maintain a proper operating temperature range in the engine;

▪ power electronics controller (inverter) to manage the flow of electric energy delivered by traction battery and to control the electric traction motor’s speed and torque;

▪ regenerative braking system for exploiting the kinetic energy to recharge the battery, during braking;

▪ electric transmission for transferring the mechanical power from electric motor to the driving wheels. Motors can also be mounted inside the wheels.

Batteries are grouped into packs that can be realized in different ways, using series or parallel connections between groups of cells. BEV motors typically operate at a few hundred volts, that’s why a minimum of about 100 cells is required (for example, considering a cell voltage of 3.6 V, 100 Li-Ion batteries can produce 360 V). Alternatively, some vehicles have many but smaller cells, up to tens of thousands. Battery pack is the largest and most expensive element of the BEV, because it is typically the unique power source. A fundamental concept for modern battery electric cars is to have a removable and replaceable battery pack: this can allow to extend the driving range through the use of swap stations, but obviously the substitution must be fast and non-complex.⁶⁴ In addition to the basic pack, thermal management and voltage-monitoring systems are required to prevent overcharging of the energy source and to detect degradation or failure. The powertrain of BEVs is simpler than that of ICE vehicles: power is transmitted to the driving wheels through few selected components, reducing the friction among mechanical elements. In addition, the rapid dynamics of electric motors enables accurate and precise control of wheel torque, giving the possibility of implementing faster, safer and more reliable stability and safety control systems – active cruise control, collision avoidance, etc. – with respect to conventional cars. 65

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Figure 1.7: Principal elements of a battery electric vehicle.

The two types of electric powertrain systems are converted and dedicated. In converted systems, an electric motor and batteries replace the ICE and fuel tank, while other components remain the same. Dedicated electric powertrains are integrated into vehicles where the body and chassis design are optimized for BEVs, taking advantage of the flexibility that electric propulsion systems can guarantee. Dedicated systems include different configurations: out-wheel where the electric motors are mounted on vehicle chassis and provide the traction forces to the driving wheels directly or through gearboxes; in-wheel where electric motors are located inside the driving wheels. In-wheel technology can minimize the mechanical components but requires motors resistant against lateral and longitudinal loads and water intrusion. Both in-wheel and out-wheel can be converted to an all-wheel drive powertrain (fig. 1.8).66 On the road, an electric car is able to proceed totally silently: to avoid problems and hazardous situations for citizens and drivers, all these vehicles must be equipped with a noise alarm to advice the presence of the car. To minimize the wasted energy and to increase consequently the autonomy range of BEV, two mechanisms are used: stopping the engine when it is stationary (idle-off condition) and recharging the battery during braking (regenerative braking system) or while traveling downhill (exploiting the potential energy).

66 Un-Noor, F., et al. (2017, August), op. cit., pp. 10-12.

\ www.teoresigroup.com

BATTERY ELECTRIC VEHICLE - BEV

• Battery : In an electric drive vehicle, the auxiliary battery provides electricity to power vehicle

accessories.

• Charge Port: The charge port allows the vehicle to connect to an external power supply in order to charge the traction battery pack.

• DC/DC converter: This device converts higher-voltage DC power from the traction battery pack to the lower- voltage DC power needed to run vehicle accessories and recharge the battery.

• Onboard charger: Takes the incoming AC electricity supplied via the charge port and converts it to DC power for charging the battery. It monitors battery characteristics such as voltage, current, temperature, and state of charge while charging the pack.

Key Components of a Full Electric Car

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22

Figure 1.8: In-wheel motor all-wheel drive on an electric powertrain (EM = electric motor).67

1.2.3 Fuel-cell electric vehicles

«Using batteries as the only energy source in BEVs is limiting in a number of ways.

Most importantly, this reliance on batteries results in limited driving ranges and extended recharging times. Recently, the application of fuel-cells as and energy source in electric vehicles has received great attention».⁶⁸ A fuel-cell electric vehicle (FCEV) uses fuel-cells to generate electricity from hydrogen fuel; for this reason, they are often called hydrogen fuel-cell vehicles. However, a fuel-cell is not a storage device – differently from batteries, ultracapacitors and flywheels. It is an energy source unit where generated electricity is provided to the traction motor, while excess energy is stored in the on-board energy storage system (for example, a battery or ultracapacitor) for future needs. In this way, the driving range of a fuel-cell electric vehicle is comparable to an ICE car.69

68 Ivi, p. 65.

69 Eberle, U., & von Helmolt, R. (2010), op. cit., p. 237.

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Figure 1.9: Fuel-cell hybrid-electric vehicle configuration.70

The electrochemical reaction converts energy stored in hydrogen to electrical energy, the efficiency of FCEVs is greater than that of internal combustion-based systems and it can reach peaks over 85%. These kinds of cars can produce their own electricity emitting no carbon and producing water as a by-product of their power generating process. The major advantage is the possibility of refilling these vehicles in the same amount of time required to fill conventional ICE vehicles.⁷¹ The principal drawback of this solution for transportation is related to the difficulties of storing hydrogen fuels: durability, reliability, cost of implementation, storage, production and delivery of hydrogen are challenging problems for automakers. In general, to facilitate the diffusion of FCEVs, a complete hydrogen infrastructure – composed of pipeline hydrogen transport and filling stations – is needed.⁷² For the production, the most common method consists of gaseous hydrogen, while the second method uses the liquid form; electrolyzing water with electricity is another solution for producing gaseous hydrogen. The method of refuelling a fuel-cell car must be safe, creating a dedicated connection between the vehicle and the station dispenser for maintaining a sealed system and avoiding hazards.73

71 Ivi, p. 6.

72 Garland, N., Papageorgopoulos, D., & Stanford, J. (2012, December), Hydrogen and fuel cell technology:

Progress, challenges, and future directions, In Energy Procedia, 28, p. 6.

73 Staffell, I., et al. (2018, December), The role of hydrogen and fuel cells in the global energy, In Energy &

Environmental Science, 12(2), p. 5.

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24 On the market, the first fuel-cell vehicle has been the FCX, developed by Honda in 1999, while the second generation FCX Clarity in 2007 can be considered the beginning of one of the more successful ventures in the hydrogen vehicle industry.⁷⁴ The distribution of the FCX Clarity has been limited to United States and Japan, where hydrogen refuelling stations have been opened; Honda recently estimated to start the mass production of these FCEVs in 2020. Also, Mercedes-Benz and General Motors have started developing and testing fuel-cell-based technology. «Hydrogen FCEVs combine the best features of battery electric vehicles – zero emissions, high efficiency, quiet operation, and long life – with the long range and fast refuelling time of ICE vehicles. If FCEVs can be developed economically, they will be general-purpose zero-emission vehicles (ZEVs) and will be an important component of a strategy for reducing dependence on oil, mitigating global warming, and improving urban air quality, at an acceptable cost».75

1.3 Hybrid-electric vehicles

In recent years hybrid-electric technology has advanced significantly: «it has now been recognized that the hybrid is the ideal transition phase between the traditional all- petroleum-fueled vehicles and the all-electric vehicles of the future. In popular concepts, a hybrid-electric vehicle (HEV) has been thought of as a combination of an internal combustion engine (ICE) and an electric motor».⁷⁶ The hybrid-electric vehicle is clearly the optimal solution to reduce the problems related to ICE and air pollution, but also to overcome the limitations of the electric car, from the autonomy to the lack of widespread systems and infrastructures of recharging stations. Hybrid-electric vehicle combines the electric motor and typically high-voltage battery – or other energy storage systems as ultra-capacitors or flywheels – of a purely electric car with the ICE of a conventional gasoline-based system.

«HEVs are primarily ICE driven cars that use an electrical drivetrain to improve mileage or for performance enhancement».⁷⁷ The electric motor, in fact, allows to use the heat engine in a more efficient way and allows also to recover energy during braking and to use it for

76 Wei, L. (2017), op. cit., p. 1.